1. Field of the Invention
This invention relates generally to tuning circuits, and is particularly directed to a tuning circuit apparatus which uses a composite varactor device.
2. Description of the Prior Art
In a prior art superheterodyne receiver shown on FIG. 1, and which uses Varicap diodes (PN semiconductor diodes whose capacitance varies with the applied voltage) or varactor diodes as tuning elements, a so-called bar-type antenna coil L1, a Varicap diode D1 and a by-pass capacitor C1 constitute an antenna tuning circuit 1. The tuning output from antenna tuning circuit 1 is supplied to a mixer circuit 3. A coil L2, a Varicap diode D2 and a by-pass capacitor C2 constitute a resonance circuit 2 for a local oscillation circuit 4. The local oscillation signal from local oscillation circuit 4 is supplied to mixer circuit 3. A channel-selection or control voltage Vc is supplied from a variable bias voltage source 5 through decoupling resistors R1 and R2 to Varicap diodes D1 and D2 so that the capacities of Varicap diodes D1 and D2 are controlled and the receiving frequency is varied.
On the other hand, in an autodyne (straight) receiver or a superheterodyne receiver in which a high frequency amplifier is provided at the input stage, the input stage thereof is constructed, for example, as shown in FIG. 2. More particularly, in FIG. 2, coil L1, Varicap diode D1 and capacitor C1 constitute an antenna tuning circuit 1A and coil L2, Varicap diode D2 and capacitor C2 constitute an interstage tuning circuit 2A. The tuning output from tuning circuit 1A is supplied through a high frequency amplifier 6 to the tuning circuit 2A in which it is further selected and then delivered. A channel selection voltage Vc is supplied through resistors R1 and R2 to Varicap diodes D1 and D2 from the variable bias voltage source 5.
When the plurality of tuning circuits 1 and 2 or 1A and 2A are provided as described above, it is necessary that the tuning circuits 1 and 2 or 1A and 2A be separated, as to high frequencies, from each other by by-pass capacitors C1 and C2 and decoupling resistors R1 and R2, and thereby prevented from interfering with each other.
It is further known to provide a composite varactor device D.sub.W which, as shown in FIG. 3, incorporates a plurality of Varicap diodes, for example, two Varicap diodes D1 and D2, formed on a common semiconductor substrate, for example, on the same semiconductor chip CP. In this case, the anode electrodes of diodes D1 and D2 are independently let out therefrom to terminals A1 and A2, respectively, while the cathode electrodes of diodes D1 and D2 are led out therefrom to a common terminal K. In such composite varactor device D.sub.W, since Varicap diodes D1 and D2 are formed adjacent to each other on the same semiconductor chip CP, the characteristics of the Varicap diodes D1 and D2 can be made equal to each other. Thus, it is possible to avoid the troublesome work of testing individually formed Varicap diodes and ranking the same according to their characteristics, so that Varicap diodes having similar characteristics can be selected and then used together in the tuning circuits of FIGS. 1 and 2. Moreover, the manufacturing cost of the composite varactor device can be advantageously reduced as compared with the cost of the individual Varicap diodes.
However, when the composite varactor device D.sub.W is used in the tuning circuits 1 and 2 or 1A and 2A shown in FIGS. 1 and 2, trouble may occur. More specifically, when the composite varactor device D.sub.W is to be used in the tuning circuits 1 and 2 or 1A and 2A of FIG. 1 or FIG. 2, the circuit configuration shown in FIG. 4 may be considered therefor. In such case, coils L1 and L2 are connected between the anode terminals A1 and A2, respectively, of Varicap diodes D1 and D2 and the ground, a by-pass capacitor C3 is connected between terminal K and the ground and a resistor R3 and a variable bias voltage source 5 are connected in series to the connection point between terminal K and by-pass capacitor C3. In the composite varactor device D.sub.W, the impedances of Varicap diodes D1 and D2, when terminal K is viewed from the cathode electrodes of Varicap diodes D1 and D2, can be made so small that interference can be prevented from occurring between Varicap diodes D.sub.1 and D.sub.2.
However, with the circuit arrangement shown in FIG. 4, a resonance current I.sub.1 of tuning circuit 1B flows through by-pass capacitor C3 and a resonance current I.sub.2 of tuning circuit 2B also flows through by-pass capacitor C3 so that, if by-pass capacitor C3 has significant impedance, interference occurs between tuning circuits 1B and 2B. Accordingly, the impedance of by-pass capacitor C3 must be made sufficiently small. This requires that the capacity of by-pass capacitor C3 be substantially large and also that the equivalent series resistace be made sufficiently small. In this case, however, when the receiving frequency is low, for example, within the medium wave band, it is difficult to completely satisfy the foregoing requirements with the result that interference occurs between tuning circuits 1B and 2B. This causes troubles, for example, in the case of the autodyne receiver, an abnormal oscillation occurs or the operation thereof becomes unstable; while, in the case of the superheterodyne receiver, the local oscillation signal is radiated through the tuning circuit 1B to the outside.
Moreover, if the capacity of the by-pass capacitor C3 in the circuit of FIG. 4 is increased, the time constant determined by capacitor C3 and resistor R3 becomes large so that changes of the Varicap diodes D1 and D2 are delayed relative to the corresponding changes in the channel selection voltage Vc. As a result, the responsiveness of the tuning operation becomes poor or sluggish.